3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology. A representative LTE network 10 is depicted in FIG. 1. In the LTE network 10, transmissions a base station (also referred to as Evolved NodeB, or eNB) 20 to one or more mobile stations (also referred to as user equipments, or UEs) 30 are sent using orthogonal frequency division multiplexing (OFDM) in the downlink. Uplink transmissions from the UEs 30 to the eNodeB 20 use DFT-spread OFDM. The eNodeBs 20 transfer data and telephony through a core network 40 to and from other networks, such as the Internet 50, the Public Switched Telephone Network (PSTN) 60, or the like.
The basic LTE physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 2, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port). An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. See 3GPP TS 36.211, §5.2.1. Typically, an antenna port corresponds to a physical antenna or a combination of physical antennas. There is one resource grid per antenna port.
LTE additionally supports Multiple-input multiple-output (MIMO) operation, in which both transmitter and receiver are equipped with multiple antenna ports, allowing for transmit diversity and closed-loop spatial multiplexing.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, with each radio frame consisting of ten equally-sized subframes of 1 ms, as illustrated in FIG. 3. A subframe is divided into two slots, each of 0.5 ms time duration. The resource allocation in LTE is described in terms of physical resource blocks (PRB), where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive (in time) resource blocks represent a resource block pair and correspond to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe, where the eNodeB 20 transmits downlink assignments/uplink grants to certain UEs 30 via the (enhanced) physical downlink control channel (PDCCH and ePDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and span (approximately) the whole system bandwidth. A UE 30 that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the UE 30. Similarly, upon receiving an uplink grant, the UE 30 knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the physical downlink shared channel (PDSCH) and in the uplink the corresponding link is referred to as the physical uplink shared channel (PUSCH).
Demodulation of sent data requires estimation of the radio channel, which is done by using transmitted reference symbols (RS), i.e., symbols known a priori by the receiver. In LTE, cell specific reference symbols (CRS) are transmitted in all downlink subframes and, in addition to assisting downlink channel estimation, they are also used for mobility measurements performed by the UEs 30. LTE also supports UE-specific RS aimed only for assisting channel estimation for demodulation purposes, referred to as demodulation reference symbols (DMRS). Because the DMRS is precoded, in MIMO operations, with the same precoding matrix as that used for the PDSCH transmission, the DMRS cannot be used to generate Channel Quality Indicator (CQI), Precoding Matrix Index (PMI), or Rank Indicator (RI) feedback values. Accordingly, another reference signal, referred to as the Channel State Information Reference Signal (CSI-RS), is cell-specific and used by UEs 30 to generate CQI, PMI, and RI. Although the CSI-RS is similar to CRS, the CSI-RS is transmitted much less frequently than CRS.
FIG. 4 illustrates how the mapping of physical control/data channels and signals can be done on resource elements within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start already at the second OFDM symbol. Since the CRS is common to all UEs 30 in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE 30. This is in contrast to UE-specific RS which means that each UE 30 has RS of its own placed in the data region of FIG. 4 as part of PDSCH.
Coordinated Multipoint (CoMP) refers to a set of techniques in LTE that enable dynamic coordination of transmission and reception over a variety of different base stations 20. CoMP utilizes the phenomenon of inter-cell interference (ICI) to improve overall quality for UEs 30, particularly at cell borders, and improve utilization of the network. The concept of a transmission point is heavily used in CoMP. In this context, a transmission point (or simply a point) corresponds to a set of antenna ports covering essentially the same geographical area in a similar manner. Thus a point might correspond to one of the sectors at a site, but it may also correspond to a site having one or more antenna ports all intending to cover a similar geographical area. Often, different points represent different sites. Antenna ports correspond to different points when they are sufficiently geographically separated and/or having antenna diagrams pointing in sufficiently different directions. Stated differently, a transmission point is a set of antenna ports that are geographically collocated.
Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where, from a scheduling point of view, each point is operated substantially independently from the other points. DL CoMP operations may include, e.g., serving a certain UE 30 from multiple points, either at different time instances or for a given subframe, on overlapping or not overlapping parts of the spectrum. Dynamic switching between transmission points serving a certain UE 30 is often referred to as dynamic point selection (DPS). Simultaneously serving a UE 30 from multiple points on overlapping resources is often referred to as joint transmission (JT). The point selection may be based, e.g., on instantaneous conditions of the channels, interference, or traffic. CoMP operations are intended to be performed, e.g., for data (PDSCH) channels and/or control channels such as ePDCCH. Because CoMP downlink transmissions to a particular UE 30 may emanate from points associated with different eNodeBs 20, the UE 30 is generally discussed herein as exchanging information with the network 10, rather than particular eNodeBs 20. Those of skill in the art will readily realize that a UE 30 may transmit information to or from the network 10 via one or more eNodeBs 30.
One of the principles guiding the design of the LTE system is transparency of the network 10 to the UE 30. In other words, the UE 30 is able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs 30 or network deployments. DMRS or UE-specific RS are employed for demodulation of data channels and possibly certain control channels (ePDCCH). UE-specific RS relieves the UE from having to know many of the properties of the transmission and thus allows flexible transmission schemes to be used from the network side. This is referred to as transmission transparency (with respect to the UE 30).
Geographical separation of RS ports implies that long term channel properties from each port towards the UE 30 are in general different. Example of such long term properties include the received power for each port, the delay spread, the Doppler spread, the received timing (i.e., the timing of the first significant channel tap), the number of significant channel taps, the frequency shift, and the Doppler spread. It is noted that transmitter impairments, such as frequency shift with respect to a nominal reference frequency and propagation delays in the equipment, affect the equivalent channel perceived by the UE. Therefore, RS ports that are physically collocated but associated with significantly different transmitter impairments may be perceived by the UE 30 as having different long term channel properties.
According to the LTE terminology, it is said that two antenna ports are quasi co-located (QCL) with respect to a certain long term channel property X when such long term channel property X may be assumed to be the same for both ports by the UE 30. Conversely, it is said that two antenna ports are not quasi co-located (QCL) with respect to a certain long term channel property X when such long term channel property X shall not be assumed to be the same for both ports by the UE 30.
UEs 30 may exploit knowledge of the QCL assumptions in a number of ways. For example, the complexity of channel estimation algorithms may be reduced by avoiding individual estimation of channel properties that are QCLed between different antenna ports. Another advantage is the possibility of extracting channel properties from certain ports which allow accurate estimation and applying them to other QCLed ports that do not allow equally good estimation. Other applications are also possible, one example being the indication of QCL assumptions between DMRS and CSI-RS. Since estimation of long term channel properties from DMRS is challenging, the DMRS QCL assumptions in LTE allow estimating selected long term channel properties from a signaled CSI-RS resource and applying them to DMRS, to aid DMRS estimation. Other UE 30 implementations might exploit QCL between CSI-RS and DMRS by jointly exploiting certain channel properties from both RS types, and applying them to aid estimation of either or both such RS types.
QCL properties are either defined in the standard or signaled by the network 10 to the UE 30, according to the deployment and propagation scenario. LTE Rel-11 defines QCL of Doppler shift and Doppler spread between CRS, CSI-RS and DMRS. Furthermore, delay spread and propagation delay are QCLed between a CSI-RS resource and DMRS. There are at least three technical problems deriving from this situation.
First, it is impossible to configure correct QCL assumptions when DMRS based transmission occurs from multiple points (i.e., joint transmission on the same resources) which are characterized by different frequency shift and/or propagation delay and/or delay spread. Second, demodulation performance degrades unnecessarily when CRS and/or CSI-RS SINR are low. Third, when compensation of all the above mentioned mismatches is required, the UE complexity increases.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.